LAGOS, Nigeria (AP) — An ethnic militia killed at least 20 police officers who launched a raid to try and arrest them in central Nigeria, a police commissioner said Wednesday.

The attack in Alakio, a village in Nasarawa state, saw the officers ambushed Tuesday when they tried to stop the gang that was forcing locals to take a blood oath, police commissioner Abayomi Akermale said. Nigeria, a nation of more than 160 million people, has some 250 ethnicities. Such ethnic militias can be major presences in communities, exacting taxes and controlling areas in some places.

Akermale said the death toll in the attack could be higher, as emergency officials and police officers only reached the area on Wednesday. The commissioner declined to offer any other specific details about the attack, other than to say those responsible were not Islamic extremists.

The violence, which occurred in a state bordering Nigeria’s central capital of Abuja, comes amid growing insecurity in the oil-rich nation. Islamic extremists, including those belonging to the radical network known as Boko Haram, have been launching increasingly bloody guerrilla attacks throughout the country’s predominantly Muslim north.

Ethnic militias, as well as criminal gangs known in Nigeria as “cults,” kill at will and kidnap others for ransom. Some gangs use traditional beliefs to instill loyalty from their followers, as well as strike fear into the local population. Such gangs also are known for using extreme violence and conducting rituals involving local witchcraft.